Apparatus and method for vapor growth

ABSTRACT

Herein disclosed is a vapor growth system, in which the number of dummy lines is reduced to decrease the number of lines led into a valve system, thereby enabling thin film growth having a good interfacial steepleness. The system comprising gas supplying lines A70, B71 and C72, which are made up of AsH 3  process gas lines A62, B65, C68 and balance lines A61, B64 and C67, respectively. The balance lines A61, B64 and C67 contributes equalization of products of the viscosity and the flow rate in the gas supplying lines A70, B71 and C72, and the dummy line 60. Only when AsH 3  (A), AsH 3  (B) and AsH 3  gases are not fed upon formation of the film growth, the dummy line 60 is connected to the main line. Whereby, the system is free from pressure fluctuation of the gas in the main line, with an arrangement of even a single dummy line.

This application is a division of application Ser. No. 07/865,426 filedApr. 9, 1992 now U.S. Pat. No. 5,308,433.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system for vapor growth and/ordeposition and a method using the same, particularly to such the systemprovided with a gas supply-exhaust line system for supplying materialgases and the like to a growth chamber and venting unnecessary gasestherefrom, and a vapor growth and/or deposition method using the system.

More specifically, the present invention concerns to a vapor growthand/or deposition system, in which a single dummy line is arranged to aplurality of process gas lines to reduce the number of valve switchingby half upon forming accumulated layers made from materials of differentcompositions so that pressure fluctuation in a main line due to thevalve switching may be suppressed, thereby providing a formation of asuper thin film having a stable crystallinity and an excellentinterfacial steepleness, besides providing a low-cost, simplifiedstructure by reducing the number of expensive block valves, and a vaporgrowth method using such the system.

2. Prior Art

A conventional crystal growth system has a structure shown in FIG. 5[refer to journal of Crystal Growth, 93,353 (1988), Suematsu, et. al.].Here, reference numeral 1 denotes a main line for supplying materialgases to a crystal growth chamber. Numeral 2 denotes a vent line fordischarging gases unnecessary for the crystal growth. Numerals 4, 6, 8,10, 12 and 14 are air valves for controlling flows of the gases to themain line 1. Numerals 5, 7, 9, 11, I 3 and 15 are another air valves forcontrolling flows of the gases to the vent line 2. Numerals 20, 22, 23,25, 26 and 28 are mass flow controllers for controlling gas flow ratesto adjust them to 100, 100, 180, 180, 90 and 90 ccm, respectively.Numeral 62 denotes a gas line A as a process gas line for supplying aprocess gas A. Numeral 60 is a dummy line A for supplying a hydrogen gaswith a flow rate equal to that in the gas line A. Numeral 65 is a gasline B which is a process gas line supplying a process gas B. Numeral 63is a dummy line for supplying a hydrogen gas with a flow rate equal tothat in the gas line B. Numeral 68 is a gas line C which is a processgas line supplying a process gas C. Numeral 66 is a dummy line C forsupplying a hydrogen gas with a flow rate equal to that in the gas lineC.

As shown in FIG. 5, the gas lines, A, B and C, and the dummy lines A, Band C are communicated with the the main line 1 and the vent line 2 viathe valves 4 to 15, respectively.

Now will be described an operation of the conventional vapor phasecrystal growth system having such the structure set forth above. Whenthe process gas A is not required for the crystal growth, the valve 6 ofthe gas line A is closed, while the valve 7 is opened. As a result, theprocess gas A is fed to the vent line so as not to contribute to thecrystal growth. Meanwhile, the valve 4 of the dummy line A running thegas of which flow rate is controlled to be equal to that in the gas lineA is opened, while the valve 5 is closed. Therefore, the hydrogen gaswhich has the same value of a flow rate as the process gas A is fed tothe main line 1. Now, if the process gas A is desired to be fed to themain line, the gas line A has to connected to the main line 1 and thegas line B has to connected to the vent line 2 at the same time. So, thevalves 4 and 7 are closed and the valves 5 and 6 are opened,simultaneously.

In consequence, a total gas flow rate of the gas supplied to the mainline 1 is kept constant for a reason why the flow rate of the gassupplied to the main and vent lines 1 and 2 should not be varied,thereby suppressing fluctuation of the gas pressure in the main line 1induced by the fluctuation of the flow rate. A stable pressure of thegas in the main line 1 becomes extremely important when a flow rate ofthe supplied process gas is very small.

In a gas supplying system failing in appropriate adjustment of the flowrates of gases supplied from dummy lines, a difference in gas flow ratebetween a process gas line and a dummy line causes a difference in gaspressure between the process gas line and a main line, when a processgas line having a smaller flow rate than the dummy gas line is connectedto the main line. This leads to backflow of the gas from the main lineto the process gas line when the process gas line has a small flow rate.In particular, it is necessary to switch the gas flows every fewseconds, when an extremely thin crystal such as a quantum well structureis produced. In this occasion, such the backflow of the gas causes a gapof the timings between the valve switching and the actually supplied gasflow. This finally results in fluctuation in every crystal compositionon the crystal interface (Hereinafter, this condition will be expressedas an inferior steepleness of a crystal, because of a slow switching ofthe gas.). A dummy line having a flow rate equal to that in a processline is required to obtain a thin crystal film excellent in steepleness,as described above.

On the other hand, the backflow of a gas is attributed to a small flowrate of the gas. So, there has been adopted a method, in which a processgas line having a small flow rate is connected to a line for a hydrogendiluent, as shown in FIG. 6, in order to keep the flow rate of the gasat a predetermined level or more.

In FIG. 6, numeral 1 also denotes a main line supplying material gasesto a crystal growth chamber, 2 a vent line exhausting the gasesunnecessary for the crystal growth. Numerals 5, 10 and 14 are air valvesfor controlling the gas flow to the main line 1, while 7, 11 and 15 areanother air valves for controlling the gas flow to the vent line 2.Numerals 21, 22, 25 and 28 are mass flow controllers for controlling thegas flows to 200, 1, 100 and 200 ccm, respectively. Numeral 51 is adiluting line A feeding a hydrogen gas for diluting the process gas A inthe line A. Numeral 65 denotes a gas line B supplying a process gas Band 68 is another gas line C feeding a process gas C.

Hereinafter will be described an operation of an another conventionalvapor phase crystal growth system having the structure shown in FIG. 6.When the process gas A is not required for the crystal growth, the valve6 of the gas line A62 is closed, while the valve 7 of the same is in anopen stale. The process gas A is, as a result, fed to the vent line soas not to contribute to the crystal growth. If a gas of a great flowrate such as arsine as a constituent gas is switched to be supplied tothe main line 1, the gas line A of a very small flow rate is directlysubjected to the influence of the pressure increase in the main line dueto the switching of the arsine gas so that the gas running in the mainline 1 backflows into the gas line A62. To prevent this backflow of thegas from the main line, it is necessary to increase a flow rate in thegas line A62. For this purpose, a hydrogen gas is fed to the gas line Afor dilution in order to increase the flow rate of the process gas A, sothat the backflow of the gas due to the pressure increase may besuppressed. If the gas A is required for the crystal growth, the gasline A is connected to the main line. Accordingly, the valve 7 is closedand the valve 6 is opened, simultaneously.

Although a slight pressure fluctuation in the main line caused by thevalve switching occurs, this will not cause a back flow of the gasrunning in the main line to the gas line A. This, however, causes a flowrate fluctuation of the gas running in the main line. No dummy line isdisposed because such the gas line requiring a diluting gas line has asmall flow rate.

Recently, a block valve system is used as integrated valves. The blockvalve system is a compact integration of a plurality of valvespositioned as close to the main valve as possible to largely reduce adead volume in the main line in the block valve system, therebysuppressing a change of the gas compositions because of stagnation ofthe gas therein. An interfacial steepleness is attained by a rapidchange of the gas compositions. For this reason, the main line isrequired to have a dead volume as small as possible so as to prevent thestagnation of the gas.

The system having a structure shown in FIG. 5 requires the lines led tothe block valve system, of which number is two times that of the processgas lines, since each of the process line requires a dummy line. Thatis, when the gas line A is switched over to the gas line B, the valves 4to 11, 8 valves in total, must be switched so that slight difference incontrollability of the valves occurs and this results in pressurefluctuation in the main line.

Moreover, there is a demand to decrease the number of the lines in theblock valve system as much as possible, since a unit price per line ofthe block valve system is expensive.

In the structure shown in FIG. 6, on the other hand, is impossible tofabricate a super thin film requiring a high-precision because ofdevelopment of pressure and gas flow rate fluctuation in the main valve,although the block valve system includes only a small number of lines.To obtain a stable controllability relating to a film thickness of acrystal, a dummy line for stabilizing a pressure in the main gas becomesnecessary to each process line, in addition to a diluting line foradjustment of the flow rate. This results in a system having a dummyline structure same as shown in FIG. 5. Hence, there will arise the sameproblem that it is impossible to decrease the number of the lines in theblock valve system.

In consideration of these point, this invention provides a gas supplyingline made up of a process line and a balance line, which balance linesupplies an inactive gas such as a hydrogen gas necessary for giving aconstant product of the viscosity and the flow rate to the gas suppliedto the process gas line. Also such the balance lines equipped to therespective process gas lines contribute to retain the gases, running ina plurality of gas supplying lines not supplied to the growth chamber,at a constant product of the viscosity and the flow rate. Only one dummyline is required for this gas supplying group. Consequently, it isunnecessary to dispose dummy lines of [(the number of gas supplying linein a group) - 1], and the number of the block valves decreases by [(thenumber of gas supplying lines in a group) -1]. It is therefore an objectof the present invention to provide a vapor growth system, which notonly enables reduction of the number of required gas supplying lines ledto expensive block valves to about a half, but also enables formation ofa super thin film having considerably stable crystals excellent ininterfacial steepleness by reducing the number of times of the valveoperation to a half, thereby comprising gas supplying line system havinga simplified valve structure.

DESCRIPTION OF THE INVENTION

The present invention provides a vapor growth system comprising at leastone gas supplying line group comprising process gas lines for supplyingprocess gases, balance lines for supplying flow-rate adjusting gases tothe corresponding process gas lines, and a dummy line supplying aflow-rate adjusting gas, a plurality of the process gases in the processgas lines included in the same group not being supplied to the mainline, simultaneously.

The present invention also provides a vapor growth method comprising thesteps of adjusting a flow rate in the balance line to equalize productsof the viscosity and the flow rate of a gas in a dummy line and gases ingas supplying lines, supplying a process gas required for vapor growthin the gas supplying line to a main line, and supplying a gas from thedummy line to the main line only when the process gas is not suppliedfrom the process gas line to the main line.

In the above structure, there are provided a balance line to each gasline so that a product of the viscosity and the flow rate of the gas ineach the gas line becomes constant. A pressure developed in a piping isgiven by:

    .increment.P=μ.L.u/(g.d.d),

where

d: a inner diameter of a pipe or a needle valve;

g: an acceleration of gravity;

L: a length;

μ: a viscosity of a fluid; and

u: a mean velocity of the fluid.

To produce an equal pressure difference .increment.P in fluids havingdifferent viscosity, the μ.u needs to be equal. If a flow rate exceeds aReynolds number Re=d.u.ρ/μ, a pressure difference .increment.P becomes:

    .increment.P=f.L.ρ.u.u/(2.g.m),

where

f: a friction factor;

m: a hydraulic mean depth; and

ρ: a density of a fluid.

Here, f.ρ.u.u is considered to be a product of the viscosity and theflow rate, for the sake of convenience. Any gas is adoptable as a gassupplied by the balance line, so long as it is inactive. Hydrogen,nitrogen or the like is suitable, for example. By virtue of the balanceline, a product of the viscosity and the flow rate of a gas in the gassupplying line running into the valve system becomes constant. As anexample of such the gas supplying group, there is a gas line group inwhich the gases have the same constituents, but are not supplied to themain line, simultaneously. FIG. 2 shows a TEG gas supplying line group50 used for producing a quantum well structure made up of InGaAsP (λ=1.3μm) barrier layers 42 and InGaAs (λ=1.0 μm) well layers 43, and anInCaAsP (λ=1.0 μm) waveguide layer 41 on a n-InP substrate 40, shown inFIG. 3. TEG gas supplying lines AS1, B84 and C86 necessitate their flowrate 100, 160 and 30 ccm, respectively. These gases with three differentflow rates are fed from the respective TEG bubblers through therespective mass flow controllers, of which flow rates are respectivelyset to 100, 160 and 30 ccm. If the quantum well layer is formed by usinga mass flow controller to change a flow rate of a single process gasline, correspondingly to each of the layer, a good interfacialsteepleness becomes unavailable, since an adjustment period of the orderof 1 second is required until the flow rate of the mass flow controllergets stabilized, although the growth of each the layer requiresapproximately 10 seconds. Therefore, if the gases having flow rates of100, 160 and 30 ccm, respectively, are supplied, dummy lines with flowrates of 100, 160 and 30 ccm are required corresponding to therespective flow rates of the gas lines. Practically, balance lines A80,B83 and C85 of 100, 40 and 170 ccm are provided to the respective TEGgas lines to mix a hydrogen gas having the respective flow rates withthe TEG gases so that the mixed gas of 200 ccm is supplied to each ofthe gas supplying lines A87, B88 and C89, which is equal to a totalvalue of the flow rates of the TEG gas line or a value of the balanceline. In this case, gases of two different flow rate are not fed at thesame time, and a hydrogen gas of 200 cc is supplied from a dummy line 82only when the TEG gas is not supplied to the growth chamber. Inproduction of the waveguide layer and a quantum well layer, the dummyline is not connected to the growth chamber, because the TEG gas havinga suitable flow rate is changed over to be supplied for use. In theconventional system, three dummy lines are required for three TEG gaslines, thus a valve system including six lines and 12 valves becomesnecessary. According to the present invention, only one dummy line isrequired and the number of the valves drops to 2/3 of the conventionalsince the valve system includes only 4 lines and 8 valves, by equalizingthe flow rates of the gases not used simultaneously. In particular, thenumber of the dummy lines provided in the gas supplying group becomesonly one so that switching of the dummy line corresponding to theprocess lines becomes unnecessary when the vales are changed overbetween the two gas lines. Consequently, in the case of growth ofdifferent crystals, the number of valve switching may be reduced to ahalf at each crystal interface, thus pressure fluctuation in themainline is suppressed.

In a system according to the present invention, a large effect ofreduction of the valve number is given to a line of dopants. As shown inFIG. 1, DMZn, DEZn, H₂ Se, H₂ S, SiH₄, CpMg and the like are utilized asdopants in an occasion of growth of InP and InCaAsP crystals. If a dummyline is provided to each of the dopant lines, six dummy lines becomesnecessary so that 12 valves becomes necessary. However, if a hydrogengas is supplied from a balance line such as to make the respective gassupplying lines for the dopants have an equal flow rate, a block valvewith 7 lines including one dummy line becomes possible, besides, if thevalve system is provided with a block valve, it becomes possible toutilize the remaining 5 valves in the block valve for another gassupply. As a cost of the valve block per line is about three times ofthat of one mass flow controller, a total cost of the block valve systemincluding additional mass flow controllers becomes 70% of theconventional. In the above description, although the block valve systemis employed for switching the valves of the gases fed to the growthchamber, the present invention is also effective when a cost of thevalve is expensive compared with that of a mass flow controller.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will becomes more apparent from the following detaileddescription taken in with the accompanying drawings in which:

FIG. 1 is a schematic diagram showing a vent-supply line system forgases in a vapor phase crystal growth system according to an embodimentof this invention;

FIG. 2 is an enlarged diagram of a TEG line group in the vent-supplyline system shown in FIG. 1;

FIG. 3 shows a laser structure produced by using the vapor phase crystalgrowth system according to the embodiment;

FIG. 4 is an enlarged diagram of an AsH₃ group in the vent-supply linesystem shown in FIG. 1;

FIG. 5 is a schematic diagram showing a part of a structure of avent-supply line system for gases in a conventional vapor phase growthsystem;

FIG. 5 is a schematic diagram showing a part of a structure of avent-supply line system for gases in another conventional vapor phasegrowth system.

FIG. 7 shows a sequence of valve switching in the vapor phase crystalgrowth system according to the embodiment; and.

FIG. 8 shows a sequence of valve switching in the conventional vaporphase crystal growth system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows a gas line structure of a vapor phase crystal growth systemof an embodiment according to the present invention.

In FIG. 1, numeral 1 is a main line for supplying material gases to acrystal growth chamber, while numeral 2 is a vent line for exhaustingunnecessary gases for the crystal growth. Numeral 4 denotes air valvesfor controlling gas flow to the main line 1, while numeral 5 denotesanother air valves for controlling gas flow to the vent line 2. Numeral3 is a block valve system in which the air valves 4 and 5 areincorporated close to each other on a block-like platform. Numeral 10denotes mass flow controllers controlling gas flows. There are alsoshown a TEG group 50, an AsH₃ group 51, a TMIn group 52, a PH₃ group 53and a dopant group 54.

To grow crystals to form an InGaAs/InGaAsP MQW laser structure, acrystal growth system is required, equipped with the block valve systemconnected to the AsH₃ group, PH₃ group, TEG (triethylgarium) group, theTMI (trimethylindium) group and the dopant group. Namely, there are anAsH₃ group, a PH₃ group, a TEG group and a TMI group, gases included inthe respective groups have different flow rates and are not fed to themain line, simultaneously. There are also various dopant groups, since aP-type dopant and a N-type dopant are not doped at the same time. Theseprocess gases such as TEGa and the like are mixed and adjusted bybalance lines so as to have the same flow rate as the another suppliedgases included in the same group. As a result, to continuously growdifferent kinds of crystals, gases having required concentrations forthe respective crystals are supplied through valve switching between gassupplying lines of the gases. In this case, it is unneccesary to changeover to a valve of a dummy line for adjustment of the flow rate, since aproduct of the viscosity and the flow rate in a supplying line isequalized to the another supplying lines. The dummy line is effectivelyconnected to the main line only when a crystal not made from the gas inthe main line is grown so that pressure fluctuation in the main line maybe prevented.

Hereinafter, an explanation will be made of the gas supplying group 51or supplying the AsH₃ gas, representatively, which gas with a tendencyto bring about pressure fluctuation due to a larger viscosity than thoseof another supplied gas groups causes difficulty in attainment of a goodinterface in the conventional method.

The AsH₃ group 51 consists of a balance line A61, an AsH₃ (A) processline 62 (hereinafter abbreviated as PL), a dummy line 50, a balance lineB64, an AsH₃ (B) PL65, a balance line C57, and an AsH₃ (C) PL68. Thebalance line A61 together with the AsH₃ (A) PL62 forms a gas supplyingline A70, the balance line B64 together with the AsH₃ (B) PL65 forms agas supplying line B71, and the balance line C67 together with the AsH₃(C) PL68 forms a gas supplying line C72.

As shown in FIG. 4, the flow rates of the mas flow controllers 20, 21,22, 24, 25, 27 and 28 are set to 300, 181, 77, 15, 200, 260 and 20 ccm,respectively.

Hereinafter will be illustrated an operation relating to the gas flow inthe AsH₃ group 51. Now, 77 ccm of a 20% AsH₃ gas diluted with hydrogensupplied from a cylinder is fed to the AsH₃ (A) PL62 and 181 ccm of ahydrogen gas is fed to the balance line A61. These gases are mixed inthe supplying line A70. Similarly, 200 ccm of a 20% AsH₃ gas is fed tothe AsH₃ (B) PL65, and 15 ccm of a hydrogen gas is fed to the balanceline B64. These gases are mixed in the gas supplying line B71. 20 ccm ofa 10% of AsH₃ gas is fed to the AsH₃ (C) PL58, and 260 ccm of a hydrogengas is fed to the balance line C57. These gases are mixed in the gassupplying line C72. The flow rates in the balance lines A61, B64 and C67are set so as to give an equal product of the viscosity and the flowrate to the gas supplying lines A70, B71 and C72, and the dummy line 60.

Now, a reason why the AsH₃ (A) 62, AsH₃ (B) 65 and AsH₃ (C) 68 arerespectively set to three different flow rates will be described. In thecase of a continuous growth of crystals with different compositions, ifone process line is installed and its flow rate thereof is alteredaccording to the composition of the grown crystal, there will benecessitate a considerable time until the flow rate in a mass flowcontroller get stabilized. This will further result in an inferiorinterfacial steepleness. Consequently, crystals excellent in interfacialsteepleness may be obtained by providing independent lines which are setdifferent flow rates necessary for the respective crystals and byswitching the lines at the time of a start of forming a crystaldifferent from the preceding, simultaneously.

Now will be described a valve controlling method for forming a laserstructure shown in FIG. 3, referring to FIG. 7. It is necessary tochange the flow rates of the TEGa and AsH₃ correspondingly to therespective crystals in order to form a quantum well structure comprisinga well layer 43 and a barrier layer 42. In particular, a controllingmethod of the valves in the AsH₃ will be here described, in detail.

FIG. 3 shows a laser structure including a n-InGaAsP (α=1.0 μm)waveguide layer 41, a quantum well structure made up of InGaAsP (λ=1.3μm) barrier layers 42 and InGaAs well layers 43, and a p-InP claddinglayer 44, on an InP single crystal substrate 40.

Detailed descriptions will be hereinafter made of control of the AsH₃gas upon crystal growth to form the laser structure set forth above. TheAsH₃ gases having different compositions are required when the waveguidelayer 41, the barrier layer 42 and the well layer 43 are successivelyformed. The control of the AsH₃ is performed upon production of thequantum well structure comprising the InGaAsP (λ=1.3 μm) barrier layer42 and the InGaAs well layer 43, as follows. As particularly shown inFIG. 4, 20 ccm of a 10% AsH₃ gas is required as a process gas C whengrowing the InGaAsP waveguide layer 41, 77 ccm of a 20% AsH₃ is requiredas a process gas A when growing the InGaAsP barrier layer 42, and 200ccm of a 20% AsH₃ is required as a process gas A when growing the InGaAswell layer 43. The respective gases are supplied in order for 8 minutesand 34 seconds, for 20 seconds and for 8 seconds, respectively, whenthickness of the layers are respectively determined to 150, 10 and 6rim. If the flow rate is changed in a single process line using a singlemass flow controller, a time period of the order of 2 seconds isrequired until the flow rate becomes stable. This time period iscomparable to an accumulating time of five atomic layers so that thesteepleness of the interface becomes deteriorated. It is, S accordingly,necessary to set three different flow rates to the process lines A, Band C for the ASH₃. That is, the gas A corresponds to 77 ccm of a 20%AsH₃, the gas B to 200 ccm of a 20% AsH₃ and the gas C to 20 ccm of a10% AsH₃, as described before.

In order to make products of the viscosity and the flow rate in the gassupplying lines A70, B71 and C72 constant, flow rates of hydrogen gasesin the balance lines A61, B64 and C67 are set to 181, 15 and 260 ccm,respectively. A total of the flow rates of the mixed gases arecalculated 258, 215 and 280 ccm, respectively. At this time, a flow rateof the hydrogen gas in the dummy line is set to 300 ccm. A reason whythe flow rate of the mixed gases differs from that of the dummy line isthat the AsH₃ is viscous. This viscosity of the gas largely affects whena gas, such as PH₃ or AsH₃, of a flow rate exceeding 10 ccm and a highconcentration is flowed. For this reason, a 100% PH₃ becomes about 179ccm for a 300 ccm dummy line.

FIG. 7 shows switching of the valves. When increasing the temperature ofthe substrate prior to the growth of the waveguide layer, only the PH₃is fed. At this time, the main line is supplied hydrogen from the dummyline 60 since the AsH₃ is unnecessary. The gas supplying lines A70, B71and C72 are connected to the vent line. The valves 4, 7, 11 and 15 areopened, while the valves 5, 6, 10 and 14 are closed. After increasingthe temperature, the waveguide layer 41 is formed. At that time, thedummy line is required to be connected to the vent line, besides theline C72 is connected to the main line. That is, the valves 4 and 15 areclosed and, simultaneously, the valves 5 and 14 are opened. Next, thebarrier layer is formed. The gas supplying line C72 is required to beconnected to the vent line, besides the gas supplying line A70 isconnected to the main line. That is, the valves 7 and 14 are closed and,simultaneously, the valves 6 and 15 are opened. Next, the well layer 43is formed. The gas supplying line A 70 is connected to the vent line,while the gas supplying line B71 is connected to the main line. Thevalves 6 and 11 are closed and, simultaneously, the valves 7 and 10 areopened. Thereafter, switching between the line A and the line B arerepeated ten-odd times until the growth of the quantum well structure iscompleted. In the above operation, there is no difference among productsof the viscosity and the flow rate of the gases supplied to both themain line and the vent line so that pressure fluctuation in the mainline due to the switching of the valves will be hardly occurred.

In the dopant group 54, supply of hydrogen from a balance line gives aneffect similar to a diluting line, if a flow rate of a supplied gas suchas H₂ Se 94, which is a doping gas, is extremely small, for example, 15ccm. As a doping gas, 134 ccm of a 200 ppm DMZn gas is fed to a gassupplying line A91, 64 ccm of a hydrogen gas being supplied to a balanceline 90. From the dummy line 92, 200 ccm of a 100% hydrogen gas issupplied. A total flow rate of the DMZn and the hydrogen gases for thebalancing becomes 198 ccm. This is a value of the flow rate which hasbeen reduced a value of a flow rate corresponding to a pressuredeveloped by a resistance of flow of the DMZn gas in the main linebecause of its viscosity. To the gas supplying line B94, 15 ccm of a 100ppm H₂ Se gas is supplied, while 185 ccm of a hydrogen gas is suppliedto the balance line 93. When the n-InGaAsP layer doped H₂ Se is formedafter the thermal treatment of the n-InP substrate, the flow rate of thePH₃ gas is, largely, changed from 300 ccm to 100 ccm. At this time, theflow rate of the gas in the main line is fluctuated in a slight degreeduring a time period from 0.05 to 0.07 second, which is required for theswitching of the gases. As a result, there is a possibility of backflowof the gas from the main line if installing a single line for H₂ Se witha small flow rate in the system. Such the backflow may be prevented bysetting a flow rate 185 ccm to the balance line similar to the anotherdoping gas lines so that a flow rate of the gas supplied to the mainline becomes 200 ccm, in total.

FIG. 8 shows a sequence of switching of the valves in the conventionalsystem shown in FIG. 5. It can be seen from FIG. 8, the number of timesof the valve operation according to the present invention may be reducedto a half comparing with the conventional system, during the repeatinggrowth of the barrier layers 42 and well layers 43 reaching ten-oddtimes, where an importance is given, particularly, to the interfacialsteepleness.

In the above description, the main line 1 and the vent line 2 is in areduced pressure, a 200 Torr, and the growing chamber is in a reducedpressure of 70 Torr. As a result, the pressure fluctuation in the mainline and the vent line decreases from 10 to 2 Torr or less, and theinterfacial steepleness of the crystals is reduced from 1.5 nm to 0.5 nmor less.

According to this embodiment, the balance lines are provided to therespective gas supplying lines to equalize products of the viscosity andthe flow rate in the supplying gas lines, in which process gases are notfed simultaneously, and a single dummy line is equipped although it hasbeen required to the each process line in the convention system, wherebythe number of valves may be decreased and the crystal properties and theinterfacial steepleness may be improved.

Although the above description has been made by way of the AsH₃ gas linegroup, mainly, any process gas group is adoptable so long as it includesprocess gases not fed simultaneously. Also, the present invention isadoptable to another vapor-phase growth of a crystal, such as GaAs's,another chemical compound semiconductors, oxides, insulating films,deposition and the like, in addition to InP's. Other than hydrogensupplied to the balance and dummy lines in this embodiment, anotherinactive gas may be adopted. Moreover, although the block valve systemis used as switching valves for the gases supplied to the growth chamberin this embodiment, general valves or valves having another structuremay be employed. Furthermore, organometallic gases are used in thisembodiment, another process gases for manufacturing semi conductors andsuperconductive material such as hydride, silane, and the like can beused.

As readily understood from the above description, the present inventionmay provide a vapor growth system, which enables formation of a superthin film with an excellent interfacial steepleness by reducing thenumber of the time of valve switching during the formation of the films.In addition, attainment of reduction of the number of required lines inthe expensive valve system realizes a low-cost system with a simplifiedstructure, thus bringing noticeable advantages, in practise.

What is claimed is:
 1. A vapor growth system comprising:a main gas lineconnected to a vapor growth chamber; at least one group of gas supplyinglines comprising; a plurality of process gas lines for supplying processgas, a plurality of balance lines for supplying flow rate adjustinggases to corresponding process gas lines, and a dummy line supplying aflow-rate adjusting gas; means to regulate the flow of gas from allindividual lines to said main gas line; a plurality of process gases inthe process gas lines of a single group of said gas supplying lineswhich are not supplied to the main gas line simultaneously.
 2. A vaporgrowth system according to claim 1, wherein products of the viscosityand the flow rate of gases in respective process gas lines of each ofsaid at least one group of gas supplying lines are adjusted to be equalto a product of the viscosity and the flow rate of gas in the respectivedummy line, by the corresponding balance lines.